Semiconductor devices, such as integrated circuit (IC) packages, typically include one or more semiconductor devices arranged on a lead frame or carrier. The semiconductor device is attached to the lead frame and bond wires or other electrical connections are attached to contacts on the semiconductor devices and to leads on the carrier to provide electrical interconnections between the various semiconductor devices and/or between a semiconductor device and the carrier. The device is then typically encapsulated in a plastic housing to provide protection and form a housing from which the leads extend.
The semiconductor device may be mounted on and electrically connected to the chip carrier, for example, by a diffusion soldering technique. Diffusion soldering has the advantage that the intermetallic phases of the bond produced have a higher melting point than the temperature at which the bond was produced. Therefore, the bond is stable during subsequent processing steps and, in particular, subsequent soft soldering processes.
Copper is a common material for lead frames. The high thermal coefficient of expansion (CTE) of the copper lead frame, however, can affect the accuracy of placement of small semiconductor devices on the lead frame when a diffusion soldering process is used, since the soldering procedure takes place at about 350° C., for example. When the lead frame cools, it contracts and thus can change the position of the semiconductor devices on the lead frame.
To address this, lead frames have been fabricated using materials with a lower CTE than copper, such as Alloy 42 or ceramic materials. However, these materials have a lower thermal conductivity than copper, and thus may not provide adequate heat dissipation.